Provide an attribute value pre-processing mechanism

Details

Description

Before storing/propagating/synchronizing a specific attribute it should be called an attribute value pre-processor in order to permit to manipulate each attribute value.

The idea is to provide an interface to be implemented into the overlay in order to perform custom pre-processing operations.
The custom implementation class name could be defined into the global configuration.

This new feature would give the "handle" to provide several attribute value manipulation like specific value translations, or something else.